1. Field of the Invention
The present invention relates to a square type enclosed storage battery of medium to large capacity, and more particularly to a technology for realizing high volume capacity density or high output density.
2. Description of the Prior Art
Hitherto, the demand for secondary battery was mainly occupied by the small cells for use in power source of portable appliances such as camcorder, and the development has been mainly promoted in the direction of smaller size and larger capacity. Recently, there is an increasing demand for high performance secondary batteries of medium capacity or large capacity, ranging from scores to hundreds of Ah, such as the stationary secondary battery used as no-break power unit or the emergency power source installed in computer system, and secondary battery used as a mobile power source used in electric vehicle (hereinafter called EV) and other motor-driven vehicle developed for environmental or energy measure.
Among them, for example, the secondary battery used for moving an EV or the like is required to have a high output, high energy density, and high reliability, capable of withstanding the large current load of over 100 A (amperes), in order to obtain acceleration, cruising distance and reliability equally competitive with an internal-combustion engine such as gasoline engine. In this background, to surpass the performance of the improved lead storage battery being hitherto considered for EV use, the nickel-cadmium storage battery, nickel-hydrogen storage battery, other alkaline secondary battery, and moreover, for use in future, lithium secondary battery are being researched and developed as promising batteries satisfying these requirements.
In such promising battery system of square type enclosed storage batteries of medium to large capacity, the conventional current collector structure was as shown in FIG. 3 (A), in which a lead plate 9 coming out from a electrode plate group 8 is connected to one current collector 2f projecting from a flange 2d, and an electric power (energy) accumulated in the electrode plate group 8 is taken out of the battery, and therefore if attempted to shorten the distance F from the upper end of the electrode plate group to the cover inside ceiling wall, when connecting the current collector 2f and the lead plate 9, in particular, the lead plate 9 at the outermost side of the electrode plate group 8 is deformed by force, and a short circuit occurs due to disconnected welding of the lead plate 9 with positive electrode plate 8a or negative electrode plate 8b, or twisting of the lead plate 9, and hence the distance from the electrode plate group 8 to the terminal 2 was long (Japanese Laid-open Utility Model No. 55-22971). Or, in order to connect a cell and a cell electrically, when fixing a connection plate by using a screw 2a of the terminal 2, the locking structure around the terminal 2 was only holding of the periphery of the terminal 2 at two to four positions by fixing ribs 1c of the cover 1 as shown in FIG. 3 (B) or (C).
Besides, in a terminal 2 as shown in FIG. 4, a lead plate 9 coming out from a electrode plate group 8 is connected to a current collector 2f at a position eccentric from a screw 2a, a pole portion 2b, and a sealing groove 2c of the terminal 2, and an electric power (energy) accumulated in the electrode plate group 8 is delivered to outside of the battery, and in this current collecting structure, the lead plates are gathered rationally and the distance from the electrode plate group 8 to the terminal 2 is shortened, and thereby the electrode plate occupying volume in the cell is increased and the volume capacity density is enhanced. However, the current feeding distance from the electrode plate group 8 to the terminal 2 was long (U.S. Pat. No. 5,158,842).
To realize the volume and capacity density (the energy per unit volume of cell) and output density (the output capacity per unit weight of cell) required in such storage battery of high performance, it is necessary to reduce the electric resistance value from the electrode plate group to the terminal, and to deliver the accumulated electric power (energy) to outside of the cell at a minimum limit of loss, as well as to improve the characteristic of the positive and negative plates and to study the optimum composition condition of electrode plate group using them. Moreover, a higher voltage is required for higher output, and the reliability for series connection of about 10 to 250 cells is needed.
In the current collecting structure of the conventional square type enclosed storage battery of medium to large capacity, no consideration was given to handling of large current such as discharging always at 100 A or more and charging by regenerative brake of an equal capacity, or reliability for series connection of 10 to 250 cells in order to obtain high voltage. Therefore, in the conventional current collecting structure as shown in FIG. 3 (A), since the current feeding distance from the electrode plate group 8 to the terminal 2 is long, the electric resistance value is large from the terminal 2 to the lead plate 9, and the Joule heat in this area was very large.
Still more, for electric connection of a cell and a cell by using the screw 2a of the terminal 2, in the procedure of series connection of hundreds of cells, a torque of 250 to 300 kgfcm may be applied, while the usual coupling torque ranges from 70 to 200 kgfcm, due to fluctuation of coupling torque (excessive torque) by mechanical work, and such torque may cause to rotate the terminal 2 or dislocate the terminal 2 from the lead plate 9, or the screw is loosened by deformation of connection plate to be coupled by increase of shaft power of screw coupling, thereby increasing the electric resistance value. Or, in the terminal 2 shown in FIG. 4, in the storage battery of the current collecting structure in which the lead plate 9 coming out from the electrode plate group 8 is connected to the current collector 2f at a position eccentric from the screw 2a, pole portion 2b, and sealing groove 2c of the terminal 2, since the annular packing 3 is not pressed uniformly, there is also a problem of electrolyte leak, and since it is used in high voltage state in a narrow space of EV or the like, if electrolyte leaks from the terminal, even in some cells, the leak current may lead to discharge, short circuit, heat generation, fire or electric shock.
It is hence not applicable to the battery for EV or the like handling large current such as discharging always at 100 A or more and charging by regenerative brake of an equal capacity, in which high volume capacity density, high output density, long life, and high reliability are required.
Reduction of temperature rise of battery by lowering of electric resistance value brings about an improvement of charging efficiency for the alkaline secondary battery which is inferior in charging efficiency in high temperature atmosphere, and also brings about an improvement of battery life characteristic by suppressing deterioration of electrode plate active substance due to heat. This is because in the battery for EV handling large current such as discharging always at 100 A or more and charging by regenerative brake of an equal capacity (in the EV, in order to utilize the energy effectively, the motor works as generator when applying the brake, and the generated electric energy is charged in the storage battery, which is characteristic of this system), the temperature rise of battery due to Joule heat (=square of current.times.electric resistance; the electric resistance being proportional to the length of conductor and inversely proportional to the sectional area) is very significant. The EV is often charged right after running. It means that the battery is charged in high temperature state after the temperature is raised by running. Further, if the electrode plate compound is always held at high temperature, deterioration of the compound is accelerated, and the life of the storage battery is shortened. Therefore, suppression of battery temperature rise by lowering of Joule heat is very important for improving the charging efficiency and enhancing the battery life.